The secondary vitreous of the adult, which seems nonessential in its normal state, can be a highly significant structure when diseased, dramatically demonstrating its importance in health and disease. The vitreous has several highly specific functions such as the molecular sieve effect, prevention of vasoproliferation, and modification of inflammatory and immunological processes. These functions are closely connected to its macromolecular composition, its physical state (gel or liquid), its structural integrity, and, ultimately, its metabolism. This investigation will focus on the in vivo metabolism and regeneration of gel and liquid vitreous. It will attempt to localize these processes by selective degeneration of retinal structures and implantation of cultured vitreous cells. Histological and autoradiographic studies will provide insight into the metabolism of vitreous cells, retinal structures, their regenerative properties, and the interrelation of the cells with the vitreous. The amount and nature of newly formed vitreous will be determined by appropriate chemical analyses and by studying the incorporation of radioactive precursors. Complementing these studies, there will be a series of in vitro experiments to analyze the metabolism of the vitreous cells, using chemical, pulse labelling, and autoradiographic techniques. The effect of the extracellular vitreous matrix and its components on the biosynthetic and proliferative properties of the cells will be determined.